Biaxial film process and rotary apparatus therefor

ABSTRACT

In a biaxial blown tube synthetic resin film making apparatus, the extruder die, mandrel, the reheat ovens, and a hot air ring are caused to rotate and oscillate about a stalk moving coaxially therethrough to provide an improved flatter and more uniform film.

This is a division of application Ser. No. 794,042, filed May 5, 1977,now U.S. Pat. No. 4,112,034.

BACKGROUND OF THE INVENTION

This invention relates to a blown tube process and apparatus forproducing biaxially oriented synthetic thin gauge resin film such aspolypropylene film, and more particularly to a blown tube apparatuswhere the extruder die, reheat ovens, and hot air ring are caused torotate and oscillate in unison to provide a flatter, more uniformpolypropylene film.

In a well-known biaxially oriented blown tube apparatus and process,polypropylene resin is fed through an extruder die head to form acontinuously moving cooled and solidified tubular section or stalk whichthen passes through a reheat furnace to bring it up to a predeterminedorientation temperature. At the predetermined orienting or softeningtemperature, the stalk is insufflated or expanded into a very large thingauge cylindrical structure referred to as a blown tube or bubble. Thiscylindrical structure is closed at one end by having it pass through apair of nip rolls, and thereafter the double lay flat biaxially orientedsection emanating from the nip rolls is slit and wound on storage ormill rolls.

One of the problems of such a process is that an irregular section or agauge non uniformity causes a continuing build up in the film in a millroll because the non uniformity is continually wound on itself in eachturn. Large rolls so wound have differences in roll diameters because ofthis cascading effect. These differences, because of the creepcharacteristics of the film degrade the basic flatness eveness andstraightness of the film. When precision winding machines such ascapacitor roll winding machines are utilized to wind capacitor rolls,this degradation causes skewering and wrinkling in the winding filmwhich are extremely undesirable factors.

SUMMARY OF THE INVENTION

It has now been discovered, that the extruder die and equipment utilizedto increase or reduce the temperatures in the process lending up to thepoint of insufflation of the stalk must have their effects applied ordirected against the stalk in a manner to have an effect on theuniformity of the gauge of polypropylene stalk circumferentially, inorder to provide a film with optimum physical characteristics,particularly for electrical capacitor use. In one preferred form of thisinvention, the polypropylene stalk which only moves axially passesthrough a reheat oven which coaxially surround the polypropylene stalkand is caused to rotate in unison with a combination rotating die andmandrel. At the same time, a hot air ring which is positioned near theshoulder of the insufflated tube is also caused to simultaneously rotatesynchronously with the reheat ovens and the die head. After a period ofrotation, these rotating units are caused to reverse their rotation anda process is periodically repeated to provide a form of oscillationmovement.

DESCRIPTION OF THE DRAWINGS

This invention will be better understood when taken in connection withthe following description and the drawings in which:

FIG. 1 is a schematic illustration of a blown tube apparatus of thisinvention.

FIG. 2 is a schematic illustration of the rotating die of thisinvention.

FIG. 3 is an enlarged schematic and cross sectional view of the die headrotator of FIG. 2.

FIG. 4 is an enlarged schematic and cross sectional view of the die andmandrel of FIG. 2.

FIG. 5 is a partial schematic and cross sectional illustration of therotating reheat oven, the rotating hot air ring of this invention, and abubble support system.

FIG. 6 is a schematic representation of the bubble support system for apolypropylene bubble which does not have a rotating hot air ring.

FIG. 7 is a schematic and exaggerated illustration of a polypropylenestrip of the prior art with a gauge non uniformity.

FIG. 8 is a schematic and exaggerated illustration of a polypropylenestrip of this invention with a gauge non uniformity.

PREFERRED EMBODIMENTS OF THE INVENTION

A schematic illustration of an apparatus utilized to providepolypropylene film by the blown tube process is shown in FIG. 1 asapparatus 10. In apparatus 10, polypropylene resin is fed into hopper 11from where if passes into an extruder 12 where the temperature iselevated sufficiently to provide molten characteristics to the resin.The molten resin then passes upwardly through a feed duct 13 to acombined rotating die and mandrel unit 14 of this invention. The moltenresin emanates from the die and mandrel unit as a thick walled tubesection or stalk 15. In order to provide the proper travel or axialmovement of stalk 15 through the die unit 14, a pair of tube advancerrolls 16 are employed to provide an upward pull on the stalk 15. Rolls16 neck the stalk 15 so as to close off the stalk volumes below andabove the rolls 16 around a central air pipe (not shown).

Stalk 15 then passes from the advancer rolls 16 into a rotating reheatoven unit 17 which may be a combination of one or more stacked ovens. Inthe oven 17 the temperature of the stalk 15 is raised to a predeterminedlevel where the plasticity of the polypropylene is in the desiredorientation range of the insufflation step. Stalk 15, after passingthrough the oven 17 is caused to move through a rotating hot air unit18. Hot air ring unit 18 provides currents of hot air to be directedagainst the stalk 15 to stabilize and equalize the temperature at aconstant and predetermined orientation level above that in the reheatoven. At this point, air is introduced into stalk from the central airpipe (not shown) to expand the stalk into the bubble 19, as illustrated.

After the stalk 15 has been expanded to a bubble form 19, it passesthrough a cold air ring 20 which cools the bubble to provide a fixedlevel of solidification of the polypropylene. Bubble 19 then passesupwardly through a collapse frame roller assembly 21 which tapers thebubble towards a line section through a pair of nip rolls 21. The niprolls 21 pinch off the bubble 19 to retain a given amount of airpressure therewithin. In this process the film is biaxially oriented. Itis oriented in the cross machine direction (CMD) because the expandingstalk or bubble provides lateral stretching. At the same time, the niprolls provide an axial stretching of the bubble for orientation in themachine direction (MD). The polypropylene which emanates from the niprolls emanates as a dual lay flat biaxially oriented sheet joined at theedges. At this point the polypropylene film runs through a slitting andwinding assembly 22 wherein the polypropylene strip is slit to provide apair of strips each of which are then wound on supply rolls 23, asillustrated.

An important feature of this invention is a rotating die mandrel unit14, which is more particularly illustrated in FIGS. 2 and 3. Referringnow to FIG. 2, the rotating die mandrel unit comprises a rotatingassembly 24 which includes a motor and gear box unit 25, and a chaindrive 26, which rotates the rotator 27. Rotator 27 is more particularlydescribed with respect to FIG. 3.

Referring now to FIG. 3, rotator assembly 27 is illustrated ascomprising a stationary apparatus 29 and a rotary apparatus 30. Rotaryapparatus 30 with its sprocket drive 31 is mounted for rotation onstationary apparatus 29 by means of bearings 31, and is effectivelysealed by means of seals 32. As the molten resin proceeds from theextruder 12 through duct 13, it enters the rotating die and mandrelassembly 28 as illustrated in FIG. 4.

Referring now to FIG. 4, a rotating die and mandrel assembly 28comprises a die member 34 having a cylindrical orifice 35 thereinthrough which the molten resin from duct 13 is caused to enter. Themolten resin which emanates from die member 34 passes over a cylindricalquench mandrel 36. Quench mandrel 36 contains a cooling medium, as knownin the art, for example in U.S. Pat. No. 4,008,022 to Carrico assignedto the same assignee as the present invention, and reduces thetemperature of the molten polymer emanating from the die 13 to cause itto solidify and to progressively move upwardly as a tube. In the presentinvention the mandrel 36 is connected to the die member 34. By means ofthe chain drive 26, the die 34 and the mandrel 36 are caused to rotatein unison so that the polypropylene stalk slips at the interface betweenthe stalks and the mandrel 36. By this means, gauge irregularities arecircumferentially distributed by the rotator of the die 39. Slippage ofthe stalk 15 permits the use of a stationary tube advancer (rollassembly 16) thus reducing the complexity. It furthermore avoids theneed for a rotating bubble.

The rotating reheat oven unit 17 and hot air ring 18 are best describedwith respect to FIG. 5. Referring now to FIG. 5, there is illustrated apartial and schematic view of a combined hot air ring 18 and reheat ovenunit 17. Reheat oven unit 17 includes a motor drive assembly 37 whichrotates the furnace assembly through a ring gear 38. The internalsurface of the ovens (not shown), include appropriate electrical radiantheaters arranged peripherally about the interior surface which areconnected by means of electrical cables 39 to a source of power 40 (notshown). These heaters operate in a temperature range of 700°-1200° F.and provide heating circumferentially of the stalk. Coiled electricalcables 39 are sufficiently long to permit at least 360° of rotating ofthe reheat oven 17 and are suspended on trolley means. Reheat oven 17may thus be defined as a circumferential or ring oven. As the stalk 15passes through the reheat oven, its temperature is raised to apredetermined level which is ordinarily in the range necessary forinsufflation. The temperature in the reheat ovens must be sufficientlyhigh that all points of the stalk are brought up close to the requiredtemperature. The stalk temperature is then raised by a small incrementto the orientation level, within the hot air ring structure 18.

The hot air ring structure 18 of this invention is also best describedwith respect to FIG. 5. Referring again to FIG. 5, there is illustrateda partial and cross sectional view of a rotating hot ring structure 18.Air ring structure 18 is supported on a frame or platform 41 separatefrom the reheat oven 17 for independent rotation. Hot air for the airring structure 18 is supplied from a source 42 (not shown) throughconduit 43 into an annular chamber 44. Fitted into the annular chamberis a hot air lip assembly 45. Lip assembly 45 forms a closure wall forchamber 44 and is rotationally mounted on chamber 44 through bearingmeans 46 and 47. The lip assembly 45 is driven by means of a chain drive48 from a motor source of power 49 (FIG. 1). Air from annular chamber 44passes through a series of flow distributors 50 and a specially shapedorifice 51 to be directed against stalk 15. The temperature of the airin the hot air ring 18 is from about 250° F. to 450° F. and the flowrate is from 70- 130 cub. ft. per minute with a stalk speed of fromabout 15 to 80 ft. per minute.

In addition to the rotating die 14, reheat oven 17, and hot air ring 18,an improved bubble support system 52 is employed. This support system 52is also fixed to the rotating lip assembly 45 as is illustrated inFIG. 1. Referring again to FIG. 1, the bubble support system 52 isillustrated as being interposed between the hot air ring assembly 18 andthe shoulder section 53 of the bubble 19. The function of the bubblesupport system is not only to lend some stability and support to thebubble during the operation of the process and apparatus, but also toprovide initial support during the start-up process when the bubble isfirst formed. A prior art support system is illustrated schematically inFIG. 6.

Referring now to FIG. 6, a section of the bubble 19 is shown asextending from the hot air ring 18 to the bubble shoulder 52. Directlyabove the hot air ring there is a series of cylindrical roll means 54 incircumferential and transverse relationship to bubble 19. These rollersare usually of a non-metallic or soft material such as teflon whichlightly engage the bubble and are rotated by frictional engagement withthe bubble. Usually four such rolls 54 are employed. it has beendiscovered that the optimum advantages gained by the rotary apparatusesof this invention were being detrimentally affected by the chanellingand localizing of hot air flow from hot air ring 18 around the rolls.

Channelling and localizing of this hot air has been significantlyreduced by the ball arrangement illustrated in FIG. 5. Referring againto FIG. 5, a frame assembly 55 is shown as attached to the lip assembly45 of hot air ring 18. A plurality of circumferentially mounted brackets56 support a circular ring or axle 57 which surrounds the stalk 15.Mounted for rotation on the ring is a series of about 30 teflon balls 58of about 1.0 inch diameter. The balls 58 are rotated by being infrictional engagement with tube 19 and because of their number andshape, they provide better distribution and equalization of the hot airfrom the hot air ring 18. An improvement in film gauge is obtainedthrough the use of this rotatin air ring.

The rotating system of this invention comprises three units i.e., (1)die and mandrel unit, (2) reheat oven unit of one or more ovens, and a(3) hot air ring unit (including the bubble support) which arepreferably operated in unison and are synchronized with respect to speedand origin. The rotational speed of each of the three units is adjustedto the same value at a common point in their rotation. The units thenrotate at the same speed for 360° of travel. At that point a suitablecontrol such as a microswitch for each unit is tripped and the unit iscaused to move 360° in the reverse direction wherein a furthermicroswitch is tripped and the rotational cycle is then repeated.

The electrical circuit which interconnects the three units is designedso that in the event one of the units trips its microswitch prior to thetripping of the other switches, there will be a delay in operation untilthe other microswitches are tripped. Then all three units being thereverse cycle simultaneously. This arrangement provides sufficientalthough not precise synchronization.

It has been discovered that operation, i.e., oscillation of the rotaryunits of this invention without synchronization unexpectedly actuallyreinforces film gauge irregularities at various positions about theperiphery of the bubble 19. However, oscillation even withoutsynchronization provides more uniformity than would be the case if noneof the units oscillated. There are, however, various combinations ofoscillations which may be used in the practice of this invention.Preferably a 360° max travel is employed to obtain better results.

In one practice of this invention a rotational speed of the rotatingunits may fall in the range of 4 to 15 minutes per revolution. A speedof 6 minutes per revolution gave good results with polypropylene filmand approximates 5 revolutions of the entire bubble for one bubble.

The practice of this invention provides enhanced control over the manyvariables in the bubble process of manufacturing biaxially orientedpolypropylene film and improves the final product, particularly withrespect to roll uniformity and flatness of the film as well as withrespect to bag and camber. Thick and thin spots in a film strip as aresult of gauge variations are very detrimental to electrical gradefilms used in capacitors. The improved process steps of this inventionare directed to the rotation or oscillation of the noted three units toperform their simultaneous heat treating and smoothing functionscircumferential of the moving stalk. Initially the die and mandrelrotates while the stalk does not rotate, but moves axially from the die.Then the reheat ovens apply a uniform exposure to heat because of therotating reheat ovens. And finally, a hot air ring provides uniformdrafts of hot air circumferentially of the axially moving stalk. Allunits must be carefully coaxially aligned to prevent hot spots.

The practitioner has a choice of changing the many process variablessuch as flow rate, resin temperature, die opening, etc., to change thequality of the final film. However at the point where this inventionstarts these variables have been controlled to a very precise degree,and yet the minor variations which occur and which are indeed small whenviewed alone, build up rapidly on a more or less standard roll which mayweigh about 500 to 1000 lbs., be 4 to 7 feet wide and contain at least60,000 ft. of film. In some bubble processes of the non biaxiallyoriented kind, it has been known to rotate a die head or even the niprolls and other upper components. The marked improvements of the presentinvention are obtained by the rotation of the temperature relatedcomponents of the process, i.e., die and cooling mandrel, reheat ovenand hot air ring, and their rotation and oscillation at predeterminedrates.

The practice of this invention also provides a trouble indicator in thefilm making process. For example, in the prior process the major if notsolely mechanically adjustable item was the die which was adjusted toprovide a thicker or more uniform thickness final film. When anirregularity occurred in the final film, the die was usually adjusted toaccommodate the irregularity whenever else it occurred in the system,usually in an unknown location. If, for example, the irregularityoccurred in the reheat oven, oscillation of the oven units could verywell magnify the problem or create a more serious problem with the othervariables in the system. By the same token, oscillation of one variablenot contributing to the problem could also magnify the problem if itwere occurring later in the cycle. By the present invention, theoperation is provided with selection means whereby he may change one ormore variables to find the variable which is causing a problem.

Many variations of the mounting and drive means of this invention willbecome apparent to those skilled in the art. All units may be separatelymounted or mounted for integral rotation. Several intermediatecombinations are possible. Drive means may follow the same format.

In prior biaxial film processes where a gauge non uniformity appeared,it became rolled up on itself over thousands of feet of film in a rolluntil a significant diameter difference is noted. After storage for onlya short period of time, the creep characteristics of the film cause aset in the film such that upon unrolling there is a distinct unevenessin the film. When such a roll is placed in a winding machine there arevarious detrimental results, notably wrinkling of the film in thewinding machine because the irregularity causes variation in rate offeed of the film from the longer or smaller roll diameter embodying thenon uniformity. Where the film has a camber, because for example of aridge in the roll diameter at one end of a roll, the straight feed on awinding machine causes wrinkling of the film being taken off the roll.

The present invention is a unique defect handling and distributorarrangement. To some degree, because of its temperature effects it tendsto ameliorate some film irregularities. To a greater extent, itdistributes the irregularities throughout the film strip or bubblesurface. It operates in one respect to remove the registration effect offilm irregularities in a roll and to distribute them laterally over aroll.

This distribution ecomes even more important in film metallizingprocesses where a film web is passed through an evacuated and heatedchamber wherein aluminum is vacuum deposited on the film. Wrinkling andbagginess in the film is exceedingly difficult to handle in such aprocess. The film of the present invention lays flat under these extremeconditions because of its unique non uniformity distribution structure.

For example, referring now to FIG. 7, there is schematically illustrateda partly unrolled section 59 of the film of the prior art. In the filmsection 59, the line 61 denotes a line of a non uniformity, for examplean area of a thicker film. This area 61 is axial, i.e., in the machinedirection and is wound up on itself. In FIG. 8 illustrating the filmsection 60 of this invention, the same non uniformity now numbered 62 isdistributed laterally or, in effect, in both the machine direction andcross machine direction first in one CMD and then in the opposite CMD.Other irregularities are distributed laterally depending on the rotationand oscillation. Oscillation of 180° provides a distribution in CMD overone half the film width. A metallized layer 63 on this film striprepresents an improved electrode and dielectric for capacitor use. It ischaracterized by having linear or series irregularities, non uniformity,or defects following a bias distribution pattern which has significantlateral or diagonal directions extending over about one half or more ofthe film width in one CMD before reversing to provide a diagonal path inan opposite CMD. This film strip is much more stable when passingthrough a vacuum metallizer due to its reduced bagginess and camberwhile being unwound, passed through the vacuum chamber in an unsupportedstate and then being rewound.

While this invention has been disclosed with respect to particularembodiments thereof, numerous modifications may be made by those skilledin the art without departing from its true spirit and scope. Therefore,it is intended that the appended claims cover all such modifications andvariations which come with the true spirit and scope of the presentinvention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. As an article of manufacture, an electricalcapacitor dielectric comprising a biaxially oriented polypropylenesingle film strip having opposed longitudinal edges, said strip havingbeen longitudinally slit from a wider strip, and said slit strip havinga film defect irregularity therein which repeats itself in alongitudinal series of continuous lines progressing diagonally along thesaid slit strip in the machine direction (MD) and one cross-machinedirection, (CMD) and then reverses to a longitudinal series ofcontinuous lines along the strip in the same said machine direction (MD)and an opposite cross-machine direction (CMD) and a metallized layercoating on said slit strip.
 2. The invention as recited in claim 1wherein said lines extend from one longitudinal edge of said strip tothe opposite longitudinal edge of said strip.
 3. The invention asrecited in claim 2 wherein said lines extend further in the (MD) machinedirection as compared to (CMD) cross-machine direction.